Vacuum tube



June 12, 1934; H. A SNOW 1,962,218

VACUUM TUBE Filed April 3, 1951 Patented .lune 12, 1934 STATES UNETEiitZlS arri stier.

VACUUM TUBE Delaware Application April 3, 1931, Serial No. 527,531

4 Claims.

This invention relates to vacuum tubes and more particularly to theinternal elements thereof. g The invention is of particular utility inthe pretreatment of vacuum tubes of the thermionic type 5 containingseveral electrodes sealed in a glass envelope. These electrodes areoperated in 'a high vacuum or in an atmosphere of a selected gas, and ineither case it is necessary to preheat the electrodes during exhaustionof the envelope to remove occluded gases which otherwise would be drivenolf during the operation of the device and injuriously affect itsoperation.

Various schemes have been devised to heat these electrodes, one of thembeing to place the Whole device in a magnetic field of suiiiciently highfrequency and intensity to heat these electrodes to the desiredtemperature. A heater of this type is known as the induction or highfrequency furnace. However, the induction furnace, when used on a devicehaving the usual electrodes, does not uniformly heat the electrodes. Itis Well known that, when heating vacuum tube electrodes by placing themin the same high frequency electromagnetic iield, a large electrode israised to a higher temperature than a small electrode, both electrodesbeing made of materials having about the same specic resistance. Whenthe small electrode is placed Within the large electrode, as is commonpractice in vacuum tube design, and the electrodes placed together inthe magnetic held, the ield in proximity to the small electrode isreduced in intensity by the shielding action of the large electrode,thus resulting in a further decrease in the heating of the smallerelectrode. Y

An object of this invention is to provide a vacuum tube, the electrodesof which may be more uniformly heated by an induction furnace.

Another object is to provide a vacuum tube,

`4l() the electrodes of which are of varying resistivity.

having an internally heated cathode 1, preferably provided with an oxidecoating 2, and cylindrical cold electrodes or elements constituting acontrol grid 3,' screen grid fl, anode 5 and outer screen 6. Theseelements are metal cylinders mounted coaxial with the cathode 1, and areenclosed by an envelope '7. The envelope is exhausted to a low pressureby a suitable vacuumV pump through the exhaust tube 8.

As commonly constructed, the outer screen 6 is formed of a Woven Wirefabric of low resistance to induced circulating currents and it isreadily heated by the usual induction or high frequency furnace duringthe evacuation of the envelope. The low resistance of the outer screen6, together with its physical arrangement about the other elements,shields the plate 5 and other interior elements from the action of thehigh frequency furnace to such an extent that it is diflicult to raisethese elements to a sufliciently high tem perature to eliminate fromthem the occluded gases without endangering the outer screen 6 byoverheating or melting.

In accordance With the present invention, the exterior element orelements of the tube are given such form or are constructed of suchmaterials that the shielding effect on the interior elements ismaterially reduced, thus facilitating the heating of the interiorelements.

As shown in Figs. l and 2, the outer screen 6 has the form of a platewith staggered diamond shaped openings 9, which may be produced by`removing the material or by first cutting a metal strip and thenelongating or expanding the same, i. e., by a process similar to thatemployed in the formation of the expanded metal lath used in buildingconstruction. Asthe induced current flows circumferentially 0i the outerscreen 6, the openings 9 materially increase the length of the currentpath. A further increase in resistance to induced circulating currentsmay be secured by forming the outer screen 6 of a high resistancematerial, such as nichrome.

Where slotting of a tube'element is undesirable, as may be the case withthe anode 5, a plate or sheet of normally high resistance material maybe employed. When the next inner element, such as the screen grid 4, isformed of the usual low resistance material, no difficulty isencountered in heating the elements uniformly when the anode 5 is animperforate sheet of high resistance openings and the inherentresistance of a metal having a high specific resistance.

As shown in Fig. 3, the openings 10 which pro- Vide the elongated pathmay take the form of staggered slots of substantially rectangularoutline.

Wire screen of the general type now employed for the outer screen may beused When so woven, as shown in Fig. 4, that the Wires 11 extenddiagonally of the band from which the screen is formed. Or a screenresembling that of Figs. 1 and 2 may be formed by Weaving narrow metalribbons 12 to form a band in which the ribbons are diagonally arranged,Fig. 5.

High resistance metals or alloys may, of course, be employed to eiect afurther increase in the circumferential resistance of these severaltypes of outer elements.

From the description and illustrations given it Will be evident to thoseskilled in the art that application of this invention is not limited tothe devices shown but may be employed in connection With any processWhere it is desired to use an induction furnace to heat an element orelements which are located within or substantially enclosed by an outerelement. Various changes and modifications are contemplated as beingwithin the spirit of the invention and the scope of the fol lowingclaims.

What I claim is:

1. An electrical discharge device comprising an envelope enclosing acathode, an inner sheet metal cylinder coaxial With and surrounding saidcathode, and an outer cylinder coaxial with and surrounding said innercylinder and formed of perforated sheet metal having a greaterelectrical resistance than said inner cylinder to circulating currentsinduced in said cylinders by a high frequency electro-magnetic field.

2. An electrical discharge device comprising an envelope enclosing acathode, an inner sheet metal cylinder coaxial With and surrounding saidcathode, and an outer cylinder coaxial with and surrounding said innercylinder and formed of perforated sheet metal having its perforationsstaggered to provide tortuous conducting paths of a length greater thanthe circumference of said outer cylinder, said outer cylinder having thegreater electrical resistance to circulating currents induced in saidcylinders by a high frequency electro-magnetic field.

3. An electrical discharge device comprising a cathode, acircumferentially continuous control grid surrounding said cathode ananode of thin sheet nichrome surrounding said grid and hav ing a higherresistance to induced circulating currents than said control grid, and acircumferentially continuous metal screen surrounding said anode andforming for circulating currents a closed circuit of higher resistancethan said anode.

4. An electron discharge device comprising an electron emitting cathode,a control grid surrounding said cathode, an electrostatic screensurrounding said control grid, a cylindrical sheet metal anodesurrounding screen and having a greater resistance to inducedcirculating currents than said screen and said control grid, and ametallic electrostatic shield surrounding said anode and forming aclosed circuit for circulating currents and comprising a metal sheethaving a multiplicity of openings positioned to cause said shield tohave a greater electrical resistance than said anode.

HAROLD A. SNOW.

